Paper stock (stock) is a suspension of fiber in water. The consistency of stock is measured in the percentage of fiber therein. Stock treated in the pulp and paper industry frequently contains heavy impurities, such as scraps of metal or particles of sand. Sand and metal are often transported to the mill with raw materials such as wood or waste paper. In addition, metal may also originate from the various processing devices in the mill or may be included in waste paper in the form of staples or paper clips.
The presence of these heavy impurities in these raw materials is very detrimental to the pulp and paper making process and therefore must be removed. In particular, sand and metal cause problems by wearing out the devices utilized in pulp and paper processes. Further, these impurities accumulate and clog these devices so as to render these devices susceptible to failure.
Heavy particles are usually separated from the stock by various vortex separators which have been designed according the various applications. Two exemplary types of separators are discussed below.
A conventional low consistency separator, sometimes referred to as a cleaner, usually operates at a consistency range of less than about 1% and contains a vortex separator similar to a device referenced by numeral 10 in FIG. 1. Stock is introduced at a high speed via a tangential inlet 12 to a cylindrical portion 14 of the separator so that heavy particles accumulate at the wall thereof and move along a spiral path down into a conical portion 16 of the separator. These, particles are then ejected from the bottom of the separator via an outlet 18. The purified lighter stock which is collected around the axis of the separator 10, rises up and is discharged via an outlet 20 in the upper end of separator 10. This type of vortex separator has two disadvantages, namely it requires operation with low consistency stock and thus exhibits high energy consumption. The energy consumption is high due to the low operational consistency of the stock which inherently requires the pumping of large amounts of water through the vortex cleaner and the significant amount of pressure loss within the apparatus.
Another cleaner is generally used for separating scrap and sand from high consistency stock, in the range of 3%-5%. As illustrated in FIG. 2., high consistency cleaner 30 is identical with a conventional vortex cleaner (FIG. 1) except that it includes a rotor 32. The rotor 32 usually rotates at the upper end of a conical portion 34 of cleaner 30, close to an inlet 36 and an accept discharge 38. The rotor 32 has two functions. One of the functions is to move the stock so as to prevent, the fiber network from becoming entangled and thus interfere with the scrap and sand particles from easily moving through the suspension. The second function of the rotor 32 is to subject the stock suspension to a fast circular movement so as to create a centrifugal force which forces the heavy particles to the periphery of the separator. The drawbacks of the cleaner are its large size and high energy consumption when one strives for an adequate separation efficiency. Further, the cleaner, despite its rotor, is not capable of creating pressure. In other words, the cleaner cannot advance the stock. Consequently, the stock to be cleaned must be introduced into the cleaner at such a high pressure that the stock can upon discharge retain enough pressure so as to move to the next processing step under the urging of such pressure.
Swedish Patent 457 614 discloses a turbocyclone apparatus, in which the suspension is fed through a tangential feed inlet. The suspension flows underneath a stationary spirally formed plate, which directs the suspension in a spiral path down along the converging walls of the cyclone. Thereafter the pulp is subjected to axial forces by means of a rotating rotor for increasing the separation efficiency. The lighter particles accumulate in the center of the cyclone and flow upwards along a channel within the hollow shaft of the rotor, while the heavier particles are removed from the bottom portion of the cyclone. The shaft is provided with openings in its upper portion for allowing the lighter particles to flow therethrough and into the tangential outlet of the cyclone.
It is an object of the present invention to overcome or to minimize the problems of the prior art apparatus in separating sand and scrap from stock. The apparatus of the present invention is small and its energy consumption is low.